Note 1 - Nature of Business and Financial Condition (Details Textual) $ in Thousands		3 Months Ended		12 Months Ended
	Jun. 01, 2018	Mar. 26, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Net Income (Loss) Attributable to Parent, Total				$ (27,976)	$ (24,630)	$ (37,228)
Retained Earnings (Accumulated Deficit), Ending Balance			$ (429,326)	(429,326)	(401,350)
Cash and Cash Equivalents, at Carrying Value, Ending Balance			$ 33,734	33,734	11,553
Proceeds from Issuance of Common Stock				39,394	$ 11,044	$ 28,661
Reverse Stock Split [Member]
Stockholders' Equity Note, Stock Split, Conversion Ratio	20
At-the-market Offering [Member]
Increase (Decrease) in At-the-Market Offering of Stock				$ 84,900
Offering of Stock, Remaining Capacity, Shares | shares			44,600,000	44,600,000
Stock Issued During Period, Shares, New Issues | shares			545,313	6,936,930
Payments of Stock Issuance Costs				$ 40,300
Proceeds from Issuance of Common Stock			$ 2,500
At-the-market Offering [Member] | Subsequent Event [Member]
Stock Issued During Period, Shares, New Issues | shares		3,244,941
Proceeds from Issuance of Common Stock		$ 9,900
At-the-market Offering [Member] | Other Offering Related Expenses [Member]
Payments of Stock Issuance Costs			100	500
At-the-market Offering [Member] | Sales Agent [Member]
Payments of Stock Issuance Costs			$ 100	$ 900